As is known in the art, an alternator is an alternating current (ac) output generator. To convert the ac voltage to direct current (dc) for use in charging batteries or supplying dc loads, for example, a rectifier system is used. Sometimes, the alternator is referred to as an ac machine or more simply a machine and the combined generator/rectifier system is referred to as an alternator or an alternator system.
In many cases (including automotive alternators), a diode rectifier is used to rectify the ac voltage produced by the generator. The ac generator can be modeled as a three-phase voltage source and a set of inductors.
In a so-called wound-field machine, the output voltage or current can be controlled by varying the current in a field winding which in turn varies the ac voltage magnitudes. The advantage to this approach is the extreme simplicity and low cost of the system. One particular type of wound field machine is a so-called wound-field Lundell-type alternator. A Lundell machine is characterized by the way the rotor/field of the machine is constructed, the details of which are well-known to those of ordinary skill in the art. Significantly, the construction techniques used to manufacture Lundell-type alternators result in an ac machine which is relatively inexpensive but which has a relatively high leakage inductance or reactance. Wound-field Lundell-type alternators are almost universally used in the automotive industry primarily because they are reliable and inexpensive. One problem with wound-field Lundell-type alternators, however, is that the relatively high machine inductance strongly affects the machine performance. In particular, due to the high inductance of the Lundell machine, it exhibits heavy load regulation when used with a diode rectifier. That is, there are significant voltage drops across the machine inductances when current is drawn from the machine, and these drops increase with increasing output current and machine operating speed. Consequently, to deliver substantial current into a low dc output voltage, the ac machine voltage magnitudes have to be much larger than the dc output voltage.
For example, in a typical high-inductance automotive alternator operating at relatively high speed, the internal machine voltage magnitudes are in excess of 80 V to deliver substantial current into a 14 V dc output. This is in contrast with a low-reactance machine with a diode rectifier, in which the dc output voltage is only slightly smaller than the ac voltage magnitudes.
In order to control output voltage or current, a controlled rectifier is sometimes used instead of field control. One simple and often-used approach for controlled rectification is to replace the diodes of a diode rectifier with thyristor devices. For example, as described in J. Schaefer, Rectifier Circuits, Theory and Design, New York:
Wiley, 1965 and J. G. Kassakian, M. F. Schlecht, and G. C. Verghese, Principles of Power Electronics, New York: Addison-Wesley, 1991 thyristor devices can be used in a semi-bridge converter. With this technique, phase control (i.e. the timing of thyristor turn on with respect to the ac voltage waveform) is used to regulate the output voltage or current. One problem with this approach, however, is that it can be relatively complex from a control point of view. This is especially true when the alternator must provide a constant-voltage output.
Alternatively, rather then using phase control, control is sometimes achieved using switched-mode rectification. With the switched-mode rectification technique, fully-controllable switches are used in a pulse width modulation (PWM) fashion to produce a controlled dc output voltage from the ac input voltage. This approach, which typically utilizes a full-bridge converter circuit, often yields high performance at the expense of many fully-controlled PWM switches and complex control circuits and techniques.
One relatively simple switched-mode rectifier that has been employed for alternators attached to wind turbines is described in an article entitled "Variable Speed Operation of Permanent Magnet Alternator Wind Turbines Using a Single Switch Power Converter," by G. Venkataramanan, B. Milkovska, V. Gerez, and H. Nehrir, Journal of Solar Energy Engineering--Transactions of the ASME, Vol. 118, No. 4, November 1996, pp. 235-238. In this approach, the alternator includes a rectifier comprising a diode bridge followed by a "boost switch set" provided from a controlled switch (such as a MOSFET) and a diode. The switch is turned on and off at a relatively high frequency in a PWM fashion. This approach is utilized along with PWM switching generated by a current-control loop to simultaneously control the output current and turbine tip speed of a permanent magnet alternator. The approach is specifically applied to a low-reactance (i.e. low-inductance) permanent-magnet ac machine where the battery voltage is higher than the ac voltage waveform. It should be noted that the rectifier system is topologically the same as the Discontinuous Conduction Mode (DCM) rectifier described in an article entitled "An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers," by A. R. Prasad, P. D. Ziogas, and S. Manias, the IEEE Trans. Power Electronics, Vol. 6, No. 1, January 1991, pp. 83-92, but the operating mode and control characteristics of the single switch power converter and DCM rectifier are very different.
Another controlled rectifier approach for alternators is described in U.S. Pat. No. 5,793,625, issued Aug. 11, 1998. This patent describes a circuit which utilizes the application of boost mode regulator techniques to regulate the output of an ac source.
The source inductance becomes part of the boost mode circuit, thus avoiding the losses associated with the addition of external inductors. When a three-phase alternator is the power source, the circuit comprises a six diode, three-phase rectifier bridge, three field effect transistors (FETs) and a decoupling capacitor. The three FETs provide a short circuit impedance across the output of the power source to allow storage of energy within the source inductance. During this time, the decoupling capacitor supports the load. When the short circuit is removed, the energy stored in the inductances is delivered to the load. Because the circuit uses the integral magnetics of the ac source to provide the step-up function, a relatively efficient circuit is provided. The duty cycle of the switches (operated together) is used to regulate the alternator output voltage or current. The rectifier can thus be used to regulate the output voltage and improve the current waveforms for low-reactance machines that would otherwise operate with discontinuous phase currents.
While regulating output voltage or current with a boost circuit of this type may be useful in permanent magnet alternators having relatively low inductance characteristics, this method is not useful with alternators having a relatively large inductance characteristic and a wide operating speed range such as in wound-field Lundell-type alternators for automotive applications.
To understand this, consider that in a system which includes an alternator coupled to a boost rectifier, the output voltage is fully controllable by the boost rectifier when the internal machine voltages are the same magnitude or lower than the dc output voltage as described, for example, in the above referenced Venkataramanan paper. However, if the internal machine voltages become significantly larger than the desired dc output voltage, then the output voltage cannot be regulated by the boost rectifier independent of load without inducing unacceptably high currents in the machine. The typical automotive Lundell alternator presents this problem.
At the present, high-reactance Lundell-type alternators with diode rectifiers and field control are widely used in the automotive industry. Moreover, there is a very large infra-structure dedicated to the manufacture of Lundell-type alternators. However, design of these alternators is becoming increasingly more difficult due to continually rising power levels required in vehicles and in particular required in automobiles.
As is also known, the average electrical load in automobiles has been continuously increasing for many years. The increase in electrical load is due to the demand to provide automobiles and other vehicles with increasingly more electronics and power consuming devices such as microprocessors, electric windows and locks, electromechanical valves, and electrical outlets for cell phones, laptop computers and other devices. Such additional electronics results in a need for more electrical energy in automobiles and other vehicles.
Because of this increase in electrical load, higher power demands are being placed on automotive alternator systems. Furthermore, the increasing power levels have motivated the adoption of a new higher distribution voltage in automobiles to augment and/or replace the current 14 V distribution system. In some cases, a single high-voltage electrical system may likely be used (e.g. a 42 volt electrical system). In other cases, a dual-voltage electrical system may be used which includes a first relatively high-voltage system (e.g. a 42 V electrical system) and a second relatively low-voltage system (e.g. a 14 V electrical system.) The high-voltage electrical system will be used to power vehicle components which require a relatively large amount of power such as a starter motor of a vehicle. When retained (in the dual-voltage case), the low-voltage system will be used to power vehicle components that benefit from a low-voltage supply such as incandescent lamps and signal-level electronics.
A dual- or high-voltage system having a starter motor coupled to a high-voltage bus requires a charged high-voltage battery to start. In cases where the high-voltage battery is not fully charged or is depleted, it would be desirable to be able to charge the depleted high-voltage battery from a low-voltage source in order to provide "jump-start" capability for dual/high voltage systems. In an automobile which includes only a single high-voltage system, one may desire to transfer energy from a low-voltage power source, battery or alternator of a different vehicle to the high-voltage system. In a dual-voltage system, one may desire to transfer energy from a low-voltage battery of the dual-voltage system to the high voltage battery of the dual-voltage system or from the low-voltage battery or alternator of a different vehicle or other low-voltage source to the high voltage battery of the dual-voltage system.
It would, therefore, be desirable to provide a means by which the power output capability of an alternator can be increased. It would also be desirable to provide an alternator which is capable of efficient operation at a plurality of different voltage levels. It would be further desirable to provide an alternator which is capable of operating in a dual voltage automobile. It would be still further desirable to provide a system and technique for transferring energy from a low-voltage battery of a first vehicle or from an alternator of a second different vehicle or other source to a high-voltage bus of the first vehicle.